The invention relates to an identification system for confirming authorization for access to an object or the use of an object, in particular of a motor vehicle.
Radio-based identification systems, also referred to as RF-ID “radio frequency identification” systems, are being increasingly used, for example as a replacement for mechanical key systems, in access protection for computers or for example in automatic payment systems. An RF ID system is composed of an identification mark (referred to as code transmitter) which is also referred to as an electronic key, RF-ID tag, ID transmitter or ID card which the user carries on his person or which is arranged on an object to be identified. The code transmitter is equipped with a characteristic code (code information). This code is interrogated via a base station (referred to below as a transceiver unit) and subsequently authenticated or verified.
Various radio-based transmission technologies are possible or customary: LF systems in the frequency range of 100-300 kHz, RF systems of 433 MHz or 867 MHz and high-frequency microwave systems which usually operate at the frequencies 2.4 GHz, 5.8 GHz, 9.5 GHz or 24 GHz.
In code transmitters the distinction is made between active and passive identification. Passive identification is characterized in that the code transmitter can be interrogated continuously by the transceiver unit without the intervention of the user.
If the code transmitter is located within a certain distance range from the transceiver unit, the communication between the transceiver unit and code transmitter takes place automatically or, for example in the case of manual activation of a switching device, takes place, for example, by the activation of a door latch by the user. The restriction of the distance range results generally from the radio field attenuation.
On the other hand, in the case of an active identification system, the communication is triggered actively on the code transmitter by the user. In this context, the user must therefore usually firstly operate the code transmitter manually and then, for example, additionally activate the door latch. Therefore, for reasons of increased convenience, passive identification systems are being used to a greater extent.
A customary and favorable embodiment of code transmitters are what are referred to as backscatter code transmitters (DE 198 39 696 C2). Here, a transceiver unit emits a transmission signal (referred to below as interrogation signal) with a linear frequency-modulated carrier wave in the direction of the code transmitter. If the code transmitter receives the interrogation signal, it is reflected back in a modulated fashion but not further evaluated there internally.
At the base station, the interrogation signal and response signal are evaluated, on the one hand, in terms of correspondence of the received code information with an anticipated code information item and, on the other hand, in terms of a frequency offset (frequency difference) between the transmission frequency and reception frequency. The chronological frequency offset corresponds to a radio link travelled along (signal transit time). If the frequency difference lies within a predefined interval, the code transmitter is regarded as authorized (if the code has then also been recognized as authorized), and the desired function is triggered in the object.
A disadvantage with such identification systems is that the transmission channel can be monitored unnoticed and at, in principle, any desired time. By using a suitable device it is therefore normally possible for an attacker to access the code without authorization, and thus overcome the protective function which is actually aimed at.
In the known identification systems, an unauthorized person can place a “mirror” in the vicinity of the object and use it to reflect the interrogation signal. Since an authorized code transmitter is recognized on the basis of a slight frequency difference, and a code transmitter in the vicinity of the object is simulated, the protective function is simply overcome.